tree-optimization/101280 - revise interchange fix for PR101173

tree-optimization/101280 - revise interchange fix for PR101173

The following revises the original fix for PR101173 to correctly
check for a reversed dependence rather than disallowing a zero
distance.  It also adds a check from TSVC which asks for this
kind of interchange (but with a valid dependence).

2021-07-01  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101280
	PR tree-optimization/101173
	* gimple-loop-interchange.cc
	(tree_loop_interchange::valid_data_dependences): Revert
	previous change and instead correctly handle DDR_REVERSED_P
	dependence.

	* gcc.dg/tree-ssa/loop-interchange-16.c: New testcase.